What is meant by vendor prefixes?

Answer 1

CSS vendor prefixes, also sometimes known as or CSS browser prefixes, are a way for browser makers to add support for new CSS features before those features are fully supported in all browsers.

Answer 2

Compatibility, with old device.

https://developer.mozilla.org/en-US/docs/Glossary/Vendor_Prefix

Most common tool for checking whether compatibility :

https://caniuse.com/